A number of known control arrangements regulate the position or elevation of implements, such as plows and loader buckets, attached to or drawn by agricultural and construction vehicles, such as tractors or loaders. Such control systems generally sense the position of a three-point hitch or other implement support structure and compare this position to a command value set by an operator using a command device. Based upon this comparison, such control systems generate control signals applied to valves which control the flow of hydraulic fluid to and from an actuator configured to vertically move the hitch, along with the implement mounted on it, to the desired elevation.
The hydraulic valves, which may include a raise valve and a lower valve or a three-position directional control valve, are typically solenoid-operated valves which include electrical coils. The coils operate the valves in response to electrical control signals generated by a control system. The control signals may include pulse-width-modulated (PWM) signals applied to drivers such that the rate of movement of the actuator is proportional to the duty cycle of the control signals. Typically, however, the control signal applied to each valve includes a threshold component designed to overcome inherent deadband in the valve and fluid flow forces within the valve, such as forces created by friction or springs, in order to open the valve and allow fluid to begin to flow through the valve. Thus, the control signal applied to the valve includes both a threshold component to open the valve and a component representing the desired drop or raise rate of the valve.
The threshold value for each valve may be determined during a calibration sequence in which the control signal is continuously increased until movement of the implement is detected. The minimum PWM signal which causes movement of the implement is the threshold value. The threshold value may be determined separately for the raise and lower valves. A calibration sequence to determine the raise and lower threshold values, and a control system such as that described above, is disclosed in U.S. Pat. No. 5,472,056, commonly assigned with the present invention and incorporated herein by reference.
Known control systems, however, may experience problems which cause the implement to drop or raise in an undesirable manner, with a rate of movement exceeding a desired rate. The problem may be particularly troublesome when a heavy implement is commanded to drop at a slow drop rate. The problem may also occur at a slow drop rate. The problem may also occur when a light implement is commanded to raise at a slow rate.
The problem occurs because of valve hysteresis and the effect of implement weight on the movement rate. For example, assume a heavy implement is commanded to drop at a slow drop rate. To start implement movement, a control signal including both a threshold component and a desired drop rate component is applied to the lower valve. However, once the valve opens and the implement begins to move, the rate of descent may increase beyond the desired drop rate because the descent rate depends upon the weight of the implement. While the system can attempt to slow the drop rate by minimizing the drop rate component of the control signal, the control signal cannot become smaller than the valve's threshold value. Due to valve hysteresis, reducing the control signal to the threshold value may be insufficient to limit the flow through the valve and slow the rate of descent. Thus, the implement drops in an undesirable manner. The problem cannot be solved by decreasing the control signal to completely shut off the fluid flow through the valve because this would cause the implement to abruptly stop, thereby causing a jolt to the operator and the equipment. Stopping the implement movement may cause an error between the sensed and commanded position which would then cause movement to restart, resulting in a start-stop cycle of implement movement.
Several solutions have been proposed to adjust the threshold value during the implement movement. One such solution involves sensing when the actual implement movement deviates significantly from the intended control signal. The threshold current is reduced until the derivative of the actual implement position changes. In such a manner, the negative error is diminished and a constant rate of slow lowering of the implement may be maintained.
A second solution to valve hysteresis involves using a precision control when a load is lowered manually. This solution senses when a load is about to drop below the intended control position. The threshold current is adjusted downwardly until such motion is no longer detected, thus resulting in a constant rate of lowering the implement.
However, either of the above solutions suffers from the problem that after the lowering operation is completed, subsequent lowering operations may suffer from the same sudden drops due to valve hysteresis. Thus, a need exists for an adaptable threshold current control system which can adjust the threshold current to diminish sudden drops due to valve hysteresis for the operation of lowering or raising an implement.